1. Field of the Invention
This invention relates to vertical roller mills of the rotatable grinding table type for grinding materials such as cement raw meal.
2. Description of the Prior Art
Vertical roller mills for grinding and drying materials are well known in the art. These mills include a rotary grinding track, grinding rollers (usually three in number) positioned on the track and a housing which encases the track and the rollers. The grinding rollers are pulled downwardly toward the rotary grinding track to provide a desired grinding pressure. Raw material enters the roller mill, falls onto the grinding track and is ground between the surface of the track and the rollers. The ground material is pushed from the grinding track and carried from the mill by a gas flow.
In prior art roller mills, various pressure mechanisms are utilized to cause the rollers to exert adequate grinding pressure on the rotating grinding table. A commonly used pressure mechanism is a pressure frame mounted above the rollers and supported by the rollers. Through hydraulic and/or mechanical means, the frame exerts pressure upon the rollers which exert pressure upon the grinding table. The prior art pressure frame may also be combined with draw bars. The upper end of each draw bar is mounted by movable joints to a corresponding corner of the frame and each lower end of the draw bars is mounted in or on the foundation of the mill. Hydraulic or mechanical pistons and/or telescopic mechanisms may be mounted at the lower ends of the draw bars to produce a downwardly directed force on the frame and ensure the desired pressure of the rollers against the grinding table.
In such pressure mechanisms, the frame must be able to move in a vertical plane within the mill housing to vary the pressure of the roller against the grinding table. However, it is desirable to minimize the movement of the frame in the horizontal plane which results from horizontal turning and twisting forces exerted on the frame. The prior art pressure frame is guided in the mill housing through guideways mounted on the inner wall of the housing, adjacent each corner of the frame. Inside the guideways, both the frame and the housing are provided with vertical sliding surfaces that face each other. These surfaces act as impact plates and absorb a part of the turning and twisting movement of the frame in the horizontal plane. Guide mechanisms are positioned within the guideways and also absorb the turning and twisting forces exerted on the frame, especially in the horizontal plane. These guide mechanisms are designed as strong mountings which are attached to the inner wall of the mill housing. Consequently, the housing must be constructed of strong materials to absorb the very powerful turning and twisting that are transmitted from the pressure frame to the mountings and mill housing. Even these strong mill housings, however, are weakened because of heavy vibrations transmitted to the housing during the operation of the roller mill.
I have invented a roller mill which eliminates the above described difficulties by providing a guide mechanism which substantially reduces the detrimental effects of the twisting and turning forces upon the mill housing.